Nowadays, there exist various circuits for realizing an isolation DC/DC converter. One of them is an SRC.
FIG. 1 is a diagram illustrating a conventional SRC and a resonant current characteristic curve.
FIG. 1a illustrates the conventional SRC. The conventional SRC uses resonance of an inductor and a capacitor, and shows good conversion efficiency. The conventional SRC includes DC input power 110, four switching devices (Q1 to Q4) 120, an LC resonance circuit 130 comprised of a resonance inductor 132 and a resonance capacitor 134, a transformer 140, a bridge rectification circuit 150, a capacitor 160, and an output terminal 170. The four switching devices 120 alternatively switch the input voltage of the input power 110 to convert the DC voltage to AC voltage, and transfer the AC voltage to the LC resonance circuit 130. The LC resonance circuit 130 changes the frequency characteristic of the received AC voltage. The transformer 140 has a predetermined turn ratio, converts the AC voltage (i.e. primary voltage), which is received from the LC resonance circuit 130, to voltage of a predetermined level, and thus outputs secondary voltage. The bridge rectification circuit 150 rectifies the AC voltage induced at a secondary-side of the transformer 140 into DC voltage. The capacitor 160 filters the rectified DC voltage and the output terminal 170 outputs the filtered DC voltage. Herein, the inductor constituting the LC resonance circuit 130 may also be separately added, but increased leakage inductance of the transformer 140 may also be used.
Hereinafter, a process for performing DC/DC conversion by means of such an SRC will be briefly described. The DC voltage inputted through the input power V is converted to the spherical wave pulse voltage of AC, which has positive voltage and negative voltage alternating with each other, through periodic repetition of a process in which one pair of switching devices Q1 and Q4 are turned on during a half period and the other pair of switching devices Q2 and Q3 are turned on during the other half period.
Then, the spherical wave pulse voltage of AC is transferred to the LC resonance circuit 130 comprised of the resonance inductor 132 and the resonance capacitor 134, and the LC resonance circuit 130 stores and transfers energy.
Herein, the resonance voltage and the resonance current of the LC resonance circuit 130 have amplitudes changing according to frequencies of the applied spherical wave.
The transformer 140 converts input current to output current according to its turn ratio, transfers the output current to a secondary-side. The secondary current of the transformer 140 is rectified to DC through the bridge rectification circuit 150, is filtered by the capacitor 160, and then is outputted as output voltage through the output terminal 170.
Herein, the frequency switched through the afore-described switching devices will be referred to as a switching frequency. As such a switching frequency increases, the sizes of the capacitor 160 for output filtering, the resonance inductor 132, the resonance capacitor 134, etc., can be reduced. Accordingly, it is possible to reduce the size and area of an entire circuit. In the meantime, there exists loss due to overlap of electric current and voltage during an excessive switching interval. Such loss will be referred to as switching loss. As the switching frequency increases, the switching loss also increases and the efficiency of a circuit deteriorates.
FIG. 1b illustrates the characteristic of resonant current Ir according to a switching frequency Fsw.
If the switching frequency reaches a resonance frequency
      Fr    =          1              2        ⁢        π        ⁢                                            L              r                        ⁢                          C              r                                            ,the resonant current Ir is maximized. As the switching frequency becomes greater than the resonance frequency, the resonant current Ir is reduced. However, as the switching frequency becomes less than the resonance frequency, the resonant current Ir is reduced. Due to such a characteristic, the conventional SRC controls output by means of a Pulse Frequency Modulation (hereinafter, referred to as PFM) scheme.
However, as it can be understood in the characteristic curve of FIG. 1b, one of the disadvantages of such an SRC lies in that the resonant current does not become zero no matter how the frequency increases. That is, the SRC cannot include a no-load state, in which output current becomes zero during switching, as an operation area. In order to improve control characteristics in such a no-load state, an LLC SRC and an LCC SRC have been mainly used in industrial fields.
FIG. 2 is a circuit diagram illustrating a conventional LLC SRC and LCC SRC for improving a no-load characteristic. FIG. 2a illustrates the conventional LLC SRC, and FIG. 2b illustrates the conventional LCC SRC. As illustrated in FIGS. 2a and 2b, in the LLC SRC, a parallel inductor 210 is added in parallel with the primary winding of the transformer 140. In the LCC SRC, a parallel capacitor 220 is added in parallel with the primary winding of the transformer 140.
In such a case, entire resonance voltage is divided in proportion to impedance of each part. If the primary voltage of the transformer 140 is less than output voltage, output control in a no-load state is possible because a secondary rectifier diode is not turned on.
In the LLC SRC, the parallel inductor 210 is added in parallel with the transformer 140. In the LCC SRC, the parallel capacitor 220 is added in parallel with the transformer 140. Each of the LLC SRC and the LCC SRC has the following advantages and disadvantages.
In an actual the LLC SRC, the leakage inductance of the transformer 140 is used as the resonance inductor Lr. Further, a gap is inserted into the core of the transformer 140 and magnetizing inductance thereof is adjusted, so that the parallel inductor 210 is achieved. That is, the LLC SRC is advantageous in that no additional parts exist except for the resonance capacitor 134.
However, since electric current flowing through magnetizing inductance always flows in the primary winding of the transformer 140, the primary winding must be relatively thicker than the secondary winding. Therefore, the size of the transformer 140 increases. That is, primary-secondary winding current is not in proportion to the turn ratio.
In the LCC SRC, if the parallel capacitor 220 is disposed in the secondary winding of the transformer 140, electric current flowing in the parallel capacitor 220 flows in both the primary winding and the secondary winding of the transformer 140. Therefore, the size of the transformer 140 increases as compared with that in the LLC SRC. Accordingly, if the resonance inductor 132 is separately added and the resonance capacitor 134 is disposed in the primary winding, the transformer 140 can be optimally manufactured. However, even in such a case, since the leakage inductance of the transformer 140 is not used, a manufacturing cost may increase.
As described above, the conventional SRC does not show a no-load state, i.e. cannot control output voltage due to abnormal increase in the output voltage. Further, an SRC capable of controlling a no-load state has been currently provided, but the size of a necessary power device may increase and a manufacturing cost may increase. Accordingly, it is necessary to provide an SRC capable of controlling a no-load state even without additional separate parts, and maximizing power conversion efficiency.